1. Field of the Invention
The present invention relates to a developing ink. More particularly, the present invention relates to a recording sheet or a developer sheet.
2. Description of the Prior Art
A recording sheet is well known in which the reaction of an electron donating colorless organic compound (hereinafter referred to as a color former) with an electron attracting solid acid (hereinafter referred to as a developer) is utilized. For instance, a pressure sensitive paper is described in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 2,972,547, etc.; a heat sensitive paper is described in Japanese Patent Publication No. 4160/1968 and U.S. Pat. No. 2,939,009; and a recording element is described in German Patent Laid Open (OLS) No. 1,939,628.
In these recording sheets, a developer layer is provided on the entire surface of a support and thus a desensitizer must be print-coated on areas of the developer sheet where recording is not desired (see U.S. Pat. No. 2,777,780). However, the method using the desensitizer is disadvantageous in industrially producing the developer sheet in that a coating of the developer and of the desensitizer is required and thus the coating step is complicated.
For the purpose of solving the drawback, a method wherein the developer is print-coated only on the necessary areas of the support, has been proposed and, as a matter of fact, a developing ink containing the developer is commercially available. Generally, the developing ink comprises a developer, a binder for bonding the developer to a support and a solvent to render the ink fluid. Of these ingredients, the binder significantly affects the developing ink.
That is, when a water-soluble binder, e.g., synthetic polymers such as polyvinyl alcohol and polyvinyl pyrrolidone, and natural polymers such as starch, casein and gelatin, which are used in coating the above described developer layer on the entire surface of the support, or an aqueous binder, e.g., a latex such as a styrene-butadiene rubber latex, is print-coated (spot-printed) as a developer ink, the coated areas of the support expand and contract due to water absorption, thereby not only reducing the value of the product, but resulting in the subsequent printing being out of alignment in effecting continuous print-coating. in addition, the use of organic solvent-soluble binders such as a cellulose resin, a vinyl resin, a ketone resin, a polyamide resin and an epoxy resin has been proposed. However, the adhesion of these binders to the support is not sufficient and the developer is easily peeled from the support (formation of picking) and accumulates in a printing plate, an ink blanket and an ink roller (formation of piling).
In addition, these binders considerably reduce the develping ability, and the developing ink permeates into the back of the support and forms color simply on contact with a color former layer before the use thereof (formation of fog). Furthermore, problems arise in that the printing ink (colored ink) is not sufficiently transferred to the coated developer layer and the developer layer turns yellow.